U.S. patent application number 13/753770 was filed with the patent office on 2013-08-01 for variable resistive element, and non-volatile semiconductor memory device.
This patent application is currently assigned to ELPIDA MEMORY, INC.. The applicant listed for this patent is ELPIDA MEMORY, INC., SHARP KABUSHIKI KAISHA. Invention is credited to Kazuo AIZAWA, Isamu ASANO, Takashi NAKANO, Yukio TAMAI.
Application Number | 20130193396 13/753770 |
Document ID | / |
Family ID | 48837642 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130193396 |
Kind Code |
A1 |
NAKANO; Takashi ; et
al. |
August 1, 2013 |
VARIABLE RESISTIVE ELEMENT, AND NON-VOLATILE SEMICONDUCTOR MEMORY
DEVICE
Abstract
A variable resistive element that performs a forming action at
small current and a stable switching operation at low voltage and
small current, and a low-power consumption large-capacity
non-volatile semiconductor memory device including the element are
realized. The element includes a variable resistor between first
and second electrodes. The variable resistor includes at least two
layers, which are a resistance change layer and high-oxygen layer,
made of metal oxide or metal oxynitride. The high-oxygen layer is
inserted between the first electrode having a work function smaller
than the second electrode and the resistance change layer. The
oxygen concentration of the metal oxide of the high-oxygen layer is
adjusted such that the ratio of the oxygen composition ratio to the
metal element to stoichiometric composition becomes larger than the
ratio of the oxygen composition ratio to the metal element of the
metal oxide forming the resistance change layer to stoichiometric
composition.
Inventors: |
NAKANO; Takashi; (Osaka,
JP) ; TAMAI; Yukio; (Osaka, JP) ; ASANO;
Isamu; (Tokyo, JP) ; AIZAWA; Kazuo; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHARP KABUSHIKI KAISHA;
ELPIDA MEMORY, INC.; |
Osaka
Tokyo |
|
JP
JP |
|
|
Assignee: |
ELPIDA MEMORY, INC.
Tokyo
JP
SHARP KABUSHIKI KAISHA
Osaka
JP
|
Family ID: |
48837642 |
Appl. No.: |
13/753770 |
Filed: |
January 30, 2013 |
Current U.S.
Class: |
257/4 |
Current CPC
Class: |
H01L 45/145 20130101;
H01L 27/249 20130101; H01L 45/08 20130101; H01L 45/146 20130101;
H01L 45/1253 20130101; H01L 27/2436 20130101; H01L 45/1233
20130101; H01L 45/1625 20130101 |
Class at
Publication: |
257/4 |
International
Class: |
H01L 45/00 20060101
H01L045/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2012 |
JP |
2012-017024 |
Claims
1. A variable resistive element comprising: a variable resistor, a
first electrode, and a second electrode, the variable resistor
being sandwiched between the first electrode and the second
electrode, wherein an electric resistance between the first and
second electrodes is reversibly changed by opening and closing a
filament path, formed in the variable resistor, according to an
application of voltage between the first and second electrodes, the
first electrode and the second electrode are made of conductive
materials having different work functions, a work function of the
second electrode is larger than a work function of the first
electrode, the variable resistor includes a plurality of layers
having at least a resistance change layer and a high-oxygen layer,
the high-oxygen layer is sandwiched between the first electrode and
the resistance change layer, and a ratio of an oxygen composition
ratio to stoichiometric composition of metal oxide or metal
oxynitride forming the high-oxygen layer is larger than a ratio of
an oxygen composition ratio to stoichiometric composition of metal
oxide or metal oxynitride forming the resistance change layer.
2. The variable resistive element according to claim 1, wherein
standard free energy of formation of oxide of the metal oxide or
metal oxynitride forming the resistance change layer is smaller
than standard free energy of formation of oxide of the metal oxide
or oxynitride forming the high-oxygen layer.
3. The variable resistive element according to claim 1, wherein the
high-oxygen layer and the resistance change layer are in contact
with each other.
4. The variable resistive element according to claim 1, wherein the
resistance change layer is made of n-type metal oxide or n-type
metal oxynitride, and the high-oxygen layer is made of n-type metal
oxide or n-type metal oxynitride.
5. The variable resistive element according to claim 4, wherein the
resistance change layer or the high-oxygen layer is made of oxide
or oxynitride of a material containing at least one of Hf, Ge, Zr,
Ti, Ta, W, and Al.
6. The variable resistive element according to claim 5, wherein the
resistance change layer is made of Hf oxide (HfO.sub.X) or Zr oxide
(ZrO.sub.X), wherein the stoichiometric composition ratio X of
oxygen to Hf or Zr falls within a range of
1.7.ltoreq.X.ltoreq.1.97.
7. The variable resistive element according to claim 1, wherein the
first electrode is made of a conductive material having a work
function smaller than 4.5 eV, and the second electrode is made of a
conductive material having a work function not less than 4.5
eV.
8. The variable resistive element according to claim 1, wherein the
first electrode includes any one of conductive materials of
transition metals of Ti, Ta, Hf, and Zr.
9. The variable resistive element according to claim 1, wherein the
second electrode includes any one of conductive materials of Ti
nitride, Ti oxynitride, Ta nitride, Ta oxynitride, titanium
aluminum nitride, W, WN.sub.X, Ru, RuO.sub.X, Ir, IrO.sub.X, and
ITO.
10. The variable resistive element according to claim 1, wherein an
oxide layer or oxynitride layer of the conductive material forming
the first electrode or the second electrode is formed on the first
electrode or the second electrode that is in contact with the
variable resistor through the oxide layer or oxynitride layer.
11. A non-volatile semiconductor memory device comprising: a memory
cell array including a plurality of variable resistive elements
according to claim 1 arranged in at least one of a row direction
and a column direction.
12. A non-volatile semiconductor memory device comprising: a
three-dimensional memory cell array including a plurality of
variable resistive elements according to claim 1 arranged in a row
direction, in a column direction, and in a third direction
perpendicular to the row direction and the column direction.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This Nonprovisional application claims priority under 35
U.S.C. .sctn.119(a) on Patent Application No. 2012-017024 filed in
Japan on Jan. 30, 2012 the entire contents of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a variable resistive
element storing information based upon an electric operating
characteristic in which a resistance changes due to application of
electric stress, and to a non-volatile semiconductor memory device
using the variable resistive element.
[0004] 2. Description of the Related Art
[0005] A non-volatile memory represented by a flash memory has
widely been used for a computer, communication, measuring device,
automatic control device, and device for daily use in a personal
life, as a high-capacity and compact information recording medium.
A demand for an inexpensive and high-capacity non-volatile memory
has been extremely increased. The reason of this is as follows.
Specifically, the non-volatile memory is electrically rewritable,
and further, data is not erased even if a power supply is turned
off. From this viewpoint, it can exhibit a function as a memory
card or a cellular phone that is easy to carry, or a data storage
or a program storage that stores data as an initialization upon
starting a device in a non-volatile manner.
[0006] However, in the flash memory, it takes time to perform an
erasing action of erasing data to a logical value "0", compared to
a programming action for programming a logical value "1".
Therefore, the erasing action is performed on a block basis in
order to speed up the action. However, there arises a problem that
writing by random access cannot be performed during the erasing
action since the erasing action is performed on a block basis.
[0007] In view of this, a novel non-volatile memory alternative to
the flash memory has widely been studied in recent years. A
resistance random access memory utilizing a phenomenon in which a
resistance is changed by application of voltage to a metal oxide
film is more advantageous than the flash memory in microfabrication
limit. The resistance random access memory can also operate at low
voltage, and can write data with high speed.
[0008] Therefore, research and development have actively been made
in recent years (e.g., see National Publication of Japanese
Translation of PCT Application No. 2002-537627, or H. Pagnia et al,
"Bistable Switching in Electroformed Metal-Insulator-Metal
Devices", Phys. Stat. Sol. (a), Vol. 108, pp. 11-65, 1988, and
Baek, I. G. et al, "Highly Scalable Non-volatile Resistive Memory
using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage
Pulses", IEDM 2004, pp. 587-590, 2004).
[0009] Since the programming and erasing actions can be performed
at low voltage with high speed, the resistance random access memory
using the variable resistive element having the metal oxide can
write data at an optional address with high speed. The resistance
random access memory can also use the data, which has
conventionally been used after being temporarily loaded on a DRAM,
directly from the non-volatile memory, thereby being expected to
reduce power consumption and enhance usability of a mobile
device.
[0010] As for programming and erasing characteristics of the
variable resistive element having the metal oxide, pulses having
different polarities are applied to increase (high resistance
state) or decrease (low resistance state) the electric resistance
of the element, in a driving method called bipolar switching.
Therefore, the variable resistive element is used as a non-volatile
memory by assigning a logical value to the respective resistance
states as data.
[0011] As illustrated in FIG. 15, a conventional variable resistive
element includes a lower electrode 103, a variable resistor 102,
and an upper electrode 101, those of which are stacked in this
order. It has the property of reversibly changing a resistance
value by applying a voltage pulse between the upper electrode 101
and the lower electrode 103. The conventional variable resistive
element extracts information, which is stored as a resistance
state, by reading the resistance value that is changed according to
the reversible resistance change.
[0012] A non-volatile semiconductor memory device includes a memory
cell array and a peripheral circuit. The memory cell array includes
a plurality of memory cells, each of which has the variable
resistive element described above, wherein the memory cells are
arranged in a matrix in a row direction and in a column direction
respectively. The peripheral circuit controls a programming action,
erasing action, and reading action of data to each memory cell in
the memory cell array. Because of the difference in the components
of the memory cell, there are a memory cell array (referred to as
"1T1R memory cell array") in which a memory cell includes one
selection transistor T and one variable resistive element R, and a
memory cell array (referred to as "1R memory cell array") in which
a memory cell includes only one variable resistive element R, as
the memory cell array described above.
[0013] Examples of the metal oxides used for the variable resistor
102 in the above variable resistive element include metal oxides
having a perovskite structure represented by praseodymium calcium
manganese oxide Pr.sub.1-xCa.sub.xMnO.sub.3(PCMO), and binary metal
oxides such as nickel oxide, titanium oxide, hafnium oxide, or
zirconium oxide.
[0014] In particular, the use of the binary metal oxides has an
advantage of easy microfabrication, and reduced cost for the
manufacture, since the binary metal oxides are made of materials
used in a conventional semiconductor production line.
[0015] In order to realize satisfactory resistance switching by
using the binary metal oxides described above, the variable
resistance element is formed to be asymmetric in which a thin film
of the metal oxide is sandwiched by metal electrodes, and an
interface between one of the metal electrodes at both ends and the
oxide becomes an ohmic junction or a state close to the ohmic
junction, while an interface between the other metal electrode and
the oxide becomes a state such as a schottky junction where a gap
of conductive carriers is caused. With this configuration, the
resistance state of the variable resistive element is changed
between the high resistance state and the low resistance state by
the application of voltage pulses having different polarities.
Accordingly, satisfactory bipolar switching can be realized.
[0016] C. Y. Lin, et al, "Effect of Top Electrode Material on
Resistive Switching Property of ZrO2 Film Devices", IEEE Electron
Device Letter, Vol. 28, No. 5, 2007, pp. 366-368 (hereinafter
referred to as Known Document 1), and S. Lee, et al, "Resistance
Switching Behavior of Hafnium Oxide Films Grown by MOCVD for Non
Volatile Memory Application", Journal of Electrochemical Society,
155, (2), H92-H96, (2008) (hereinafter referred to as Known
Document 2) describe respectively a variable resistive element
using Pt for one electrode, and satisfactory bipolar switching is
possible for zirconium oxide and hafnium oxide. In Known Document
1, the bipolar switching is realized by sandwiching the zirconium
oxide, which is deposited by sputtering, between a Pt electrode and
a Ti electrode. On the other hand, in Known Document 2, the bipolar
switching is realized by sandwiching the hafnium oxide, which is
deposited by MOCVD, between a Pt electrode and an Au electrode,
although the number of times of writing is one.
[0017] International Publication No. WO2010/004705 describes that a
stacked structure of at least two oxide hafnium (HfO.sub.X) layers,
each having a different oxygen concentration, is sandwiched between
electrodes, wherein one of two layers is the oxide hafnium layer
(0.9.ltoreq.X.ltoreq.1.6) having a large number of oxygen defects,
and the other one is the oxide hafnium layer
(1.8.ltoreq.X.ltoreq.2.0) having a small amount of oxygen defects.
This publication also describes that the resistance of the element
decreases due to the application of a positive voltage pulse to the
electrode with which the oxide hafnium layer having a large number
of oxygen defects is in contact, and increases due to the
application of a negative voltage pulse thereto. The resistance
change is considered to be caused because oxygen is collected or
diffused near an interface between the electrode and the oxide
layer having a small amount of oxygen defects.
[0018] Moreover, when the metal oxide having a relatively small
band gap such as titanium oxide is used as the metal oxide, an
electrode having a large work function such as platinum has to be
used in order to form a schottky barrier at the interface with the
electrode. On the other hand, when an oxide having a large band gap
such as hafnium oxide or zirconium oxide is used, a satisfactory
schottky barrier can be formed by using an inexpensive material
that is easy to be processed for electrodes, such as titanium
nitride (TiN), whereby a satisfactory switching characteristic can
be obtained, which is advantageous for integration.
[0019] In H. Y. Lee, et al, "Low Power and High Speed Bipolar
Switching with A Thin Reactive Ti Buffer Layer in Robust HfO.sub.2
Based RRAM" IEDM 2008, pp. 297-300, it is confirmed that
satisfactory bipolar switching is realized in a structure having
hafnium oxide that is formed by ALD (Atomic Layer Deposition) and
that is sandwiched by Ti and titanium nitride.
[0020] In order to utilize the variable resistive element using the
above metal oxide for an actual large-capacity semiconductor memory
device, the variable resistive element has to be adapted to the
leading-edge microfabrication technique. For this reason, it is
necessary that the data retained in the variable resistive element
can be written or read with the driving capacity of the minimum
transistor manufactured by the leading-edge processing technique.
Specifically, it is necessary that the resistance state of the
element is changed under the condition of a low voltage of about 1
V and low current of several tens of microamperes.
[0021] In the variable resistive element using the binary metal
oxide such as hafnium oxide described above, it is said that the
resistance change is produced by opening and closing a conductive
path (hereinafter referred to as "filament path") generated by an
oxygen defect formed in the oxide film in a filament form. The
filament path is formed as a result of a soft breakdown by limiting
current during a dielectric breakdown through voltage application
called forming.
[0022] Accordingly, the narrower the thickness of the filament path
is, the more the current required for the switching, i.e., the
current necessary for opening and closing the filament path that is
the cause of the resistance switching is reduced.
[0023] Generally, when voltage is applied to the variable resistive
element from an external power source to carry out the forming, the
lower limit of the current necessary for opening and closing the
formed filament path is about 1 mA. This is because it is difficult
to control the influence of current spike to a parasitic
capacitance during the forming.
[0024] On the other hand, when the amount of current flowing
through the variable resistive element during the forming is
limited by using a microfabricated transistor close to the variable
resistive element on the same chip, the current spike that charges
the parasitic capacitance can drastically be reduced. Therefore,
the lower limit of the current necessary for opening and closing
the formed filament path can be reduced to about 10 .mu.A to 100
.mu.A.
[0025] On the other hand, in the variable resistive element using
hafnium oxide or zirconium oxide, it is difficult to reduce the
current required for the switching to be not more than about 10
.mu.A to 100 .mu.A only by the current control by the transistor.
This is based upon the reason described below. Specifically, these
metal oxides have a band gap large enough for forming a
satisfactory schottky barrier even by a metal having a small work
function such as TiN compared to Pt. This means that the coupling
between the metal and oxygen is very strong. In order to form the
filament path, a certain level of voltage and current for breaking
the coupling between the metal and oxygen have to be applied to
move the oxygen. However, in the metal oxide having very strong
coupling between metal and oxygen, such as hafnium oxide and
zirconium oxide, the amount of applied voltage and current
necessary for forming the filament path is large. Therefore, it is
difficult to form a small filament path, which means it is
difficult to reduce the switching current.
[0026] A solid line in FIG. 16 indicates a change in a breakdown
current that is a lower limit of a limited current for the forming
with respect to a stoichiometric composition ratio X of oxygen to
hafnium in hafnium oxide HfO.sub.X. A broken line in FIG. 16
indicates a duration of the voltage pulse needed for a set
operation (an operation for decreasing the resistance) through the
application of the voltage pulse of 2.0 V with the current being
limited to 20 .mu.A or less.
[0027] It is found from FIG. 16 that the duration of the voltage
pulse needed for the set operation is reduced by decreasing the
stoichiometric composition ratio X of HfO.sub.X, whereby the
element can be operated with higher speed.
[0028] However, the breakdown current increases, in contrast, by
decreasing the stoichiometric composition ratio X of HfO.sub.X. As
a result, leak current (current upon dielectric breakdown) rapidly
increases. Accordingly, it is difficult to perform the forming with
small current such as in nanoampere order, so that a small filament
is difficult to form, which means it is difficult to perform
resistance switching with low voltage and small current.
[0029] When the set operation is executed with the operating
current of 20 .mu.A or less in the example in FIG. 16, the
application of the voltage pulse for about 1 .mu.s is needed for
the set operation with X being about 1.91, which means that the
high-speed operation cannot be realized.
SUMMARY OF THE INVENTION
[0030] In view of the above problems, the present invention aims to
realize a variable resistive element using a metal oxide, capable
of performing a forming process with low current and performing a
stable switching operation with low voltage and low current, and to
realize a large-capacity low-power consumption non-volatile
semiconductor memory device using the variable resistive
element.
[0031] A variable resistive element according to the present
invention for achieving the above object is characterized by
comprising:
[0032] a variable resistor, a first electrode, and a second
electrode, the variable resistor being sandwiched between the first
electrode and the second electrode, wherein
[0033] an electric resistance between the first and second
electrodes is reversibly changed by opening and closing a filament
path, formed in the variable resistor, according to an application
of voltage between the first and second electrodes,
[0034] the first electrode and the second electrode are made of
conductive materials having different work functions,
[0035] the work function of the second electrode is larger than the
work function of the first electrode,
[0036] the variable resistor includes a plurality of layers having
at least a resistance change layer and a high-oxygen layer,
[0037] the high-oxygen layer is sandwiched between the first
electrode and the resistance change layer, and
[0038] a ratio of an oxygen composition ratio to stoichiometric
composition of metal oxide or metal oxynitride forming the
high-oxygen layer is larger than a ratio of an oxygen composition
ratio to stoichiometric composition of metal oxide or metal
oxynitride forming the resistance change layer.
[0039] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that standard free energy of formation of oxide of
the metal oxide or metal oxynitride forming the resistance change
layer is smaller than standard free energy of formation of oxide of
the metal oxide or metal oxynitride forming the high-oxygen
layer.
[0040] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the high-oxygen layer and the resistance
change layer are in contact with each other.
[0041] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the resistance change layer is made of n-type
metal oxide or n-type metal oxynitride, and the high-oxygen layer
is made of n-type metal oxide or n-type metal oxynitride.
[0042] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the resistance change layer or the high-oxygen
layer is made of oxide or oxynitride of a material containing at
least one of Hf, Ge, Zr, Ti, Ta, W, and Al.
[0043] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the resistance change layer is made of Hf
oxide (HfO.sub.X) or Zr oxide (ZrO.sub.X), wherein the
stoichiometric composition ratio X of oxygen to Hf or Zr falls
within a range of 1.7.ltoreq.X.ltoreq.1.97.
[0044] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the first electrode is made of a conductive
material having a work function smaller than 4.5 eV, and the second
electrode is made of a conductive material having a work function
not less than 4.5 eV.
[0045] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the first electrode includes any one of
conductive materials of transition metals of Ti, Ta, Hf, and
Zr.
[0046] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that the second electrode includes any one of
conductive materials of Ti nitride, Ti oxynitride, Ta nitride, Ta
oxynitride, titanium aluminum nitride, W, WN.sub.X, Ru, RuO.sub.X,
Ir, IrO.sub.X, and ITO.
[0047] Moreover, the variable resistive element having the above
characteristic according to the present invention is preferably
configured such that an oxide layer or oxynitride layer of the
conductive material forming the first electrode or the second
electrode is formed on the first electrode or the second electrode
that is in contact with the variable resistor through the oxide
layer or oxynitride layer.
[0048] A non-volatile semiconductor memory device according to the
present invention for achieving the above object is characterized
by comprising a memory cell array including a plurality of variable
resistive elements having the above characteristics according to
the present invention arranged in at least one of a row direction
and a column direction.
[0049] Moreover, the non-volatile semiconductor memory device
having the above characteristic according to the present invention
is characterized by comprising a three-dimensional memory cell
array including a plurality of variable resistive elements having
the above characteristics according to the present invention
arranged in a row direction, in a column direction, and in a third
direction perpendicular to the row direction and the column
direction.
[0050] In the present invention, in the variable resistive element
having the variable resistor sandwiched between the first electrode
and the second electrode, the variable resistor includes at least
two layers that are the resistance change layer and the high-oxygen
layer, wherein the oxygen concentration is set such that the ratio
of the oxygen composition ratio to the stoichiometric composition
of the metal oxide or metal oxynitride forming the high-oxygen
layer becomes larger than the ratio of the oxygen composition ratio
to the stoichiometric composition of the metal oxide or metal
oxynitride forming the resistance change layer. This structure
facilitates to open and close a filament path, whereby voltage and
current required for a switching operation can be reduced.
[0051] It is found from a first principle calculation that energy
required to break a bond of one oxygen from hafnium oxide, which is
ideally defect-free, so as to form an oxygen defect is very high
such as 6.16 eV. On the other hand, it is found that oxygen can go
over a potential barrier with low energy such as 1.96 eV on the
shortest route in the film having the oxygen defect.
[0052] A perfect defect-free oxide is not present in nature. It has
widely been known that a stoichiometric composition ratio of
hafnium oxide or zirconium oxide is shifted toward the lack of
oxygen in nature, and hafnium oxide or zirconium oxide is
classified into an n-type metal oxide having n-type conductive
property due to an oxygen defect. Accordingly, a film grown by a
general process has an oxygen defect. Japanese Unexamined Patent
Application Publication No. 2013-004655, which was filed by one of
joint applicants of the subject application, describes that, in the
case of the hafnium oxide or zirconium oxide, in particular, oxygen
is easy to move, and the filament path is easy to be opened and
closed, by using a film having an oxygen defect, and having a
stoichiometric composition ratio X of oxygen in hafnium oxide
(HfO.sub.x) or zirconium oxide (ZrOx) falling within the range of
1.7.ltoreq.x.ltoreq.1.97 (more preferably, within the range of
1.84.ltoreq.X.ltoreq.1.92), whereby voltage and current required
for the switching operation is reduced.
[0053] Moreover, in the present invention, the oxygen ratio is made
different between the resistance change layer and the high-oxygen
layer in order to make the oxygen defect concentration of the
resistance change layer more than the oxygen defect concentration
of the high-oxygen layer. With this structure, the filament path is
opened and closed in the resistance change layer in which oxygen
easily moves due to a large number of oxygen defects, while the
filament path is always opened in the high-oxygen layer.
Accordingly, the variable resistive element that can perform a
stable switching operation with low voltage and small current can
be realized.
[0054] When the resistance change layer and the high-oxygen layer
are made of different metal oxides, the oxygen concentration
(oxygen defect concentration) is evaluated as the "ratio of the
oxygen composition ratio to the stoichiometric composition) as
described below.
[0055] For example, it is supposed that the resistance change layer
is oxide hafnium (HfO.sub.X), and the high-oxygen layer is aluminum
oxide (AlO.sub.Y). The hafnium oxide having the ideal
stoichiometric composition ratio with no oxygen defect is
HfO.sub.2. Therefore, the ratio of the oxygen composition ratio to
the stoichiometric composition of the metal oxide (HfO.sub.X)
forming the resistance change layer is X/2.
[0056] On the other hand, the aluminum oxide having the ideal
stoichiometric composition ratio with no oxygen defect is
Al.sub.2O.sub.3. Specifically, the composition ratio with no oxygen
defect is such that 3/2 oxygen atom is present for one aluminum
atom. In this case, the ratio of the oxygen composition ratio to
the stoichiometric composition of the metal oxide (AlO.sub.Y)
forming the high-oxygen layer is 2Y/3 that is obtained by dividing
Y by 3/2.
[0057] In this case, the oxygen concentration of the hafnium oxide
(HfO.sub.X) forming the resistance change layer and the oxygen
concentration of the aluminum oxide (AlO.sub.Y) forming the
high-oxygen layer are adjusted to establish X/2<2Y/3. Thus, the
variable resistive element that facilitates to open and close the
filament path and that can perform a stable switching operation
with low voltage and small current can be realized.
[0058] When the resistance change layer or the high-oxygen layer is
made of metal oxynitride (e.g., HfO.sub.XN.sub.Z), the ratio of the
oxygen composition ratio to the stoichiometric composition may be
calculated with respect to the ideal stoichiometric composition
ratio, which is HfO.sub.2, not containing nitrogen atom.
[0059] The process that can easily form a film in non-equilibrium
state, such as a sputtering method, is used for forming a film of
the metal oxide so as to form the metal oxide film that has the
oxygen composition ratio satisfying the above-mentioned condition,
whereby the resistance change layer and the high-oxygen layer can
be formed.
[0060] The variable resistive element is formed to have an
asymmetric structure in which an interface between one of the first
electrode and the second electrode and the oxide becomes an ohmic
junction or a state close to the ohmic junction, while an interface
between the other electrode and the oxide becomes a state such as a
schottky junction where energy gap of conductive carriers is
caused. With this configuration, the resistance state of the
variable resistive element is changed between the high resistance
state and the low resistance state by the application of voltage
pulses having different polarities. The filament is opened and
closed on the interface between the electrode, to which an electric
field is easily applied, and which has relatively large energy gap,
and the oxide. Therefore, the variable resistive element according
to the present invention can be realized by the structure in which
the resistance change layer is in contact with the one (here, the
second electrode), having relatively a high work function, of both
electrodes.
[0061] As a result, the variable resistive element can easily be
driven by using a microfabricated transistor having low breakdown
voltage. Consequently, the variable resistive element that can
perform a stable switching operation with low voltage and small
current can be realized. A high-integrated large-capacity
non-volatile semiconductor memory device including the variable
resistive element can easily be realized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] FIG. 1 is a schematic sectional view illustrating one
example of a structure of a variable resistive element according to
one embodiment of the present invention;
[0063] FIG. 2 is a table illustrating combinations of electrodes
that can perform resistance switching, and polarities of the
driving voltage in the switching in the variable resistive element
according to the present invention;
[0064] FIG. 3 is a graph illustrating a relationship between a flow
rate of oxygen gas added to Ar gas and a resistance value of a
formed film during film formation of an hafnium oxide film by a
sputtering method;
[0065] FIG. 4 is a view illustrating a cumulative frequency
distribution of a resistance value in a high resistance state and a
resistance value in a low resistance state after the switching
operation in the variable resistive element according to the
present invention;
[0066] FIG. 5 is an equivalent circuit diagram illustrating a
memory cell including a transistor connected in series to the
variable resistive element according to the present invention;
[0067] FIG. 6 is a view illustrating a temperature change in free
energy of formation of oxide of the metal oxide;
[0068] FIG. 7 is a view illustrating a cumulative frequency
distribution of a resistance value in a high resistance state and a
resistance value in a low resistance state after the switching
operation in the variable resistive element according to the
present invention;
[0069] FIG. 8 is a circuit block diagram illustrating a schematic
configuration of a non-volatile semiconductor memory device
according to the present invention;
[0070] FIG. 9 is a circuit diagram illustrating a schematic
configuration of a memory cell array having 1T1R structure
including the variable resistive element according to the present
invention;
[0071] FIG. 10 is a schematic sectional view illustrating one
example of a structure of a memory cell array including the
variable resistive element according to the present invention;
[0072] FIG. 11 is a perspective view illustrating one example of a
structure of a memory cell array including the variable resistive
element according to the present invention;
[0073] FIG. 12 is a schematic sectional view illustrating one
example of a structure of a memory cell array including the
variable resistive element according to the present invention;
[0074] FIG. 13 is a schematic sectional view illustrating one
example of a structure of a variable resistive element according to
one embodiment of the present invention;
[0075] FIG. 14 is a circuit diagram illustrating a schematic
configuration of a memory cell array having 1R structure including
the variable resistive element according to the present
invention;
[0076] FIG. 15 is a schematic sectional view illustrating one
example of a structure of a conventional variable resistive
element;
[0077] FIG. 16 is a graph illustrating a dependency of breakdown
current during a forming process and a duration of a voltage pulse
needed for performing a set operation (for changing a resistance
state to a low resistance state), on an oxygen composition ratio X
of hafnium oxide HfO.sub.X forming the variable resistor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0078] FIG. 1 is a sectional view schematically illustrating a
structure of a variable resistive element 1 (hereinafter
appropriately referred to as "present element 1") according to one
embodiment of the present invention.
[0079] In the drawings described below, essential parts are
emphasized for the sake of convenience of description, and a
dimensional ratio of each component of the element and an actual
dimensional ratio do not agree with each other in some cases.
[0080] The variable resistive element 1 includes a second electrode
(lower electrode) 12, a variable resistor 13, and a first electrode
(upper electrode) 14, those of which are deposited and patterned in
this order on an insulating film 11 formed on a substrate 10. The
variable resistor 13 includes at least two layers which are a
resistance change layer 15 and a high-oxygen layer 16, and each of
which is made of a metal oxide film or a metal oxynitride film.
[0081] In the present embodiment, hafnium oxide (HfO.sub.X) that
has a large bandgap and that is an insulating layer is selected to
be used for the resistance change layer 15. However, the present
invention is not limited thereto. Examples of the resistance change
layer 15 include metal oxides or oxynitrides, such as zirconium
oxide (ZrO.sub.X), titanium oxide (TiO.sub.X), tantalum oxide
(TaO.sub.X), tungsten oxide (WO.sub.X), aluminum oxide (AlO.sub.X),
germanium oxide (GeO.sub.X), hafnium oxynitride (HfO.sub.XN.sub.Z),
zirconium oxynitride (ZrO.sub.XN.sub.Z), titanium oxynitride
(TiO.sub.XN.sub.Z), tantalum oxynitride (TaO.sub.XN.sub.Z),
tungsten oxynitride (WO.sub.XN.sub.Z), aluminum oxynitride
(AlO.sub.XN.sub.Z), or germanium oxynitride (GeO.sub.XN.sub.Z).
These are n-type metal oxides or n-type metal oxynitrides.
[0082] In addition, when hafnium oxide (HfO.sub.X) is used for the
resistance change layer 15, it is preferable that its oxygen
concentration x (the stoichiometric composition ratio of oxygen to
hafnium) is adjusted to fall within the range of
1.7.ltoreq.x.ltoreq.1.97, more preferably within the range of
1.84.ltoreq.x.ltoreq.1.92.
[0083] The high-oxygen layer 16 is made of oxide or oxynitride of a
metal same as or different from the material of the resistance
change layer 15. It is configured such that the ratio of the oxygen
composition ratio to the stoichiometric composition of the metal
oxide or metal oxynitride in the high-oxygen layer 16 is larger
than that of the resistance change layer 15. In the present
embodiment, a hafnium oxide film (HfO.sub.X, wherein Y>X) that
is the metal oxide same as that for the resistance change layer 15
and that has the oxygen ratio larger than that of the resistance
change layer 15 is used for the high-oxygen layer 16.
[0084] In order to allow the variable resistive element, which is
in the initial state just after being produced, to have a state
(variable resistance state) in which the resistance state can be
changed between a high resistance state and a low resistance state
by electric stress, it is necessary to perform a so-called forming
process before the variable resistive element is used.
Specifically, in the forming process, a voltage pulse, which has a
voltage amplitude larger than that of a voltage pulse used for a
normal writing action and has a pulse width longer than that of the
same voltage pulse, is applied to the variable resistive element so
as to form a current path where resistance switching occurs in the
resistance change layer 15. It is known that a conductive path
(filament path) formed by the forming process determines the
subsequent electric property of the element.
[0085] The filament path is considered to be formed or to disappear
because oxygen atom is collected or diffused by an electric field
near an interface between the electrode and the variable resistor.
With this phenomenon, the resistance change is considered to be
caused. It is also considered that the resistance change occurs on
the interface between the variable resistor and the electrode
having large potential barrier and large work function.
Accordingly, in the present embodiment, it is supposed that the
resistance change layer 15 is in contact with the electrode (here,
the second electrode 12) having a large work function, out of the
first electrode 14 and the second electrode 12. In this case, the
resistance change layer 15 forms a schottky junction with the
electrode having a larger work function.
[0086] FIG. 2 shows the result of checking whether the resistance
switching can be executed or not with a short pulse such as 100 ns
or lower for plural variable resistive elements, each having a
different combination of the first electrode 14 and the second
electrode 12. FIG. 2 illustrates the result of a conventional
element including only the hafnium oxide film with about 3 nm as
the resistance change layer 15 without the formation of the
high-oxygen layer 16. The work function of each electrode is
described in parentheses.
[0087] As illustrated in FIG. 2, the resistance switching was not
performed in the variable resistive elements in which the first
electrode 14 and the second electrode 12 were made of the same
material.
[0088] On the other hand, when the pulse that made the second
electrode 12 negative with the first electrode 14 being defined as
a reference was applied to the element having the second electrode
12 made of TiN or Pt and the first electrode 14 made of Ta, the
resistance state was changed (set) from the high resistance state
to the low resistance state. When the pulse that made the second
electrode 12 positive with the first electrode 14 being defined as
a reference was applied to the same element, the resistance state
was changed (reset) from the low resistance state to the high
resistance state, and this element could realize a high-speed
switching. On the other hand, when the pulse that made the second
electrode 12 positive with the first electrode 14 being defined as
a reference was applied to the element having the second electrode
12 made of TiN and the first electrode 14 made of Pt, the
resistance state was changed (set) from the high resistance state
to the low resistance state. When the pulse that made the second
electrode 12 negative with the first electrode 14 being defined as
a reference was applied to the same element, the resistance state
was changed (reset) from the low resistance state to the high
resistance state, and this element could realize a high-speed
switching.
[0089] This result shows that the high-speed switching can be
realized when the material for the first electrode 14 and the
material for the second electrode 12 are different from each other.
From this result, it is also considered that the operating
interface mainly functioning as the resistance memory is different,
since polarity of the operating voltage is reversed between the
case where the first electrode 14 is made of Ta and the case where
the first electrode 14 is made of Pt, when the second electrode 12
is made of TiN.
[0090] In other words, the result described above shows that the
resistance change occurs on the interface with the electrode having
a larger work function. FIG. 2 illustrates the results of the
conventional variable resistive element having no high-oxygen layer
16. However, in the element 1 having the high-oxygen layer 16
according to the present invention, the same result is considered
to be obtained, so long as the high-oxygen layer 16 is inserted
between the electrode having a smaller work function and the
resistance change layer 15. Specifically, in the present element 1,
when the pulse that makes the second electrode 12, having a larger
work function, positive is applied, the resistance state is changed
(set) from the high resistance state to the low resistance state,
and when the pulse that makes the second electrode 12 negative, the
resistance state is changed (reset) from the low resistance state
to the high resistance state.
[0091] When the work function of the second electrode 12 is larger
than the work function of the first electrode 14, it is preferable
that the material of the first electrode 14 is selected from
conductive materials having a work function smaller than 4.5 eV,
while the material of the second electrode 12 is selected from
conductive materials having a work function equal to or larger than
4.5 eV. Examples of the conductive material forming the first
electrode 14 include Ti (4.1 eV), Hf (3.9 eV), and Zr (4.1 eV) in
addition to Ta described above (the value in each parenthesis
indicates a work function of the corresponding metal). Similarly,
examples of the conductive material forming the second electrode 12
include Ti oxynitride (TiO.sub.XN.sub.Z), Ta nitride (TaN.sub.Z),
Ta oxynitride (TaO.sub.XN.sub.Z), titanium aluminum nitride
(TiAlN), W, WN.sub.X, Ru, RuO.sub.X, Ir, IrO.sub.X, or ITO (Indium
Tin Oxide) in addition to Pt and TiN described above. Among the
electrode materials, the combination of Ti or Ta for the first
electrode 14 and TiN for the second electrode 12 is preferable from
the viewpoint of easiness in integration processing.
[0092] A method of producing the present element 1 will be
described below.
[0093] Firstly, a silicon oxide film with a thickness of 200 nm is
formed on a monocrystalline silicon substrate 10 as an insulation
film 11 by thermal oxidation method.
[0094] Then, a titanium nitride film with a thickness of 100 nm is
formed on the silicon oxide film 11 as the material for the second
electrode 12 by a sputtering process. Examples of the material for
the second electrode may include, in addition to titanium nitride
(TiN: 4.7 eV) or titanium oxynitride, tantalum nitride
(TaN.sub.X:4.05-5.4 eV), tantalum oxynitride, titanium aluminum
nitride, W (4.5 eV), tungsten nitride WN.sub.X (4.6-5.0 eV), Ru
(4.68 eV), RuO.sub.X (5.0-5.1 eV), Ir (5.35 eV), IrO.sub.X (4.2-5.2
eV), or ITO (4.5-4.8 eV), those of which has relatively a large
work function, and is popularly used in an LSI manufacturing
process. The work function of each metal is described in
parentheses.
[0095] Thereafter, a hafnium oxide film with a thickness of 2 to 5
nm (here, 3 nm) as the material for the resistance change layer 15
and a hafnium oxide film with a thickness of 2 to 5 nm (here, 3 nm)
as the material for the high-oxygen layer 16 are continuously
formed on the titanium nitride film 12, for example, by a
sputtering process. In this case, the oxygen defect concentration
of the high-oxygen layer 16 is controlled to be lower than the
oxygen defect concentration of the resistance change layer 15 by
controlling the sputtering formation ambient.
[0096] Thereafter, a tantalum film with a thickness of 150 nm is
formed as the material of the first electrode 14 on the high-oxygen
layer 16 by the sputtering process, for example. Finally, a pattern
by a photoresist process is formed, and an element region with 5
.mu.m.times.5 .mu.m is formed by dry etching as illustrated in FIG.
1, for example. Thus, the present element 1 is formed.
[0097] During the manufacturing method described above, the metal
oxide films serving as the resistance change layer 15 and the
high-oxygen layer 16 are formed by reactive sputtering in which the
metal forming the metal oxide films is used as a target, and the
amount of oxygen to be added in the film-formation ambient is
intentionally increased. With this, the film having a small amount
of oxygen defects can be formed.
[0098] FIG. 3 illustrates the relationship between the oxygen
addition amount (the ratio of oxygen partial pressure to total
pressure with argon being used as diluted gas) in the
film-formation ambient and the resistance value in the formation of
the hafnium oxide film according to the reactive sputtering in
which hafnium metal is used as a target. FIG. 3 illustrates the
result in which the thickness of the metal oxide layer is 5 nm, and
the area of the element region is 50 .mu.m.times.50 .mu.m. The
resistance value greatly reduces by decreasing the oxygen addition
amount. This is because the oxygen defect increases.
[0099] In order to stack films, each having a different oxygen
defect concentration, the respective films may successively be
formed with the oxygen addition amount being changed. For example,
when titanium nitride is used as the second electrode 12, and
tantalum is used as the first electrode 14, the respective films
are formed in the order of the titanium nitride film, the hafnium
oxide film with oxygen addition amount of 8%, the hafnium oxide
film with oxygen addition amount of 20%, and the tantalum film.
Then, the formed films are processed by photolithography and
etching, whereby the variable resistive element 1 is formed. In
this case, the hafnium oxide HfO.sub.X film (X=1.85) formed first
with the oxygen addition amount of 8% becomes the resistance change
layer 15, while the hafnium oxide HfO.sub.Y film (y=2.0) formed
later with a small amount of oxygen defects becomes the high-oxygen
layer 16.
[0100] FIG. 4 illustrates a cumulative frequency distribution of a
resistance value after the set operation and a resistance value
after the reset operation after the 1000 bits of elements 1
according to the present invention, each of which has the hafnium
oxide HfO.sub.X1 film (X1=1.85) formed by the manufacturing method
described above as the resistance change layer 15, underwent the
switching operation ten times. The experiments of the resistance
switching were carried out by using a memory cell illustrated in an
equivalent circuit diagram in FIG. 5 and having a transistor T
connected in series, and voltage pulse Vd was applied from the
present element 1.
[0101] In this case, the forming operation for forming the filament
path first, and the set operation of changing the resistance state
from the high resistance state to the low resistance state were
each performed by applying voltage Vg to a gate of the transistor T
so as to limit the current flowing through the variable resistive
element, as shown in FIG. 5. In the present embodiment, the drive
current of the transistor T was limited to 50 .mu.A, and the
voltage pulse of +3.0 V was applied for 100 ns during the forming
operation. During the set operation for changing the resistance
state from the high resistance state to the low resistance state,
the drive current of the transistor T was limited to 50 .mu.A, and
the voltage pulse of +2.5 V was applied for 100 ns. On the other
hand, during the reset operation for changing the resistance state
from the low resistance state to the high resistance state, the
gate of the transistor T was fully opened without the limitation of
current, and the voltage pulse of -1.7 V was applied for 20 ns. In
this case, the reset current flowing through the element during the
reset operation was about 200 .mu.A.
[0102] It is found from FIG. 4 that the present element 1 realizes
stable resistance switching with the state in which the set current
is limited to be not more than 50 pA. The present element 1 can
also realize high-speed switching of about 100 ns.
[0103] On the other hand, when the drive current of the transistor
T was limited to 50 pA in the conventional variable resistive
element having only the hafnium HfO.sub.X1 oxide film (X1=1.85) as
the resistance change layer 15 without having the high-oxygen layer
16, the forming operation could not be executed. This is apparent
from FIG. 16 showing that the breakdown current becomes 1 mA or
higher when the stoichiometric composition ratio X of HfO.sub.X is
1.85.
[0104] When the forming operation is executed with the current
during the forming operation being limited to be not more than 50
.mu.A in the conventional variable resistive element having no
high-oxygen layer 16, X=1.9 or higher is needed as the oxygen
concentration of the resistance change layer 15, as is understood
from FIG. 16. However, in the present element 1, the forming
operation is possible by limiting the current during the forming
operation to be not more than 50 .mu.A, even if the resistance
change layer 15 having low oxygen concentration (X=1.85) is used,
since the present element 1 includes the high-oxygen layer 16.
Therefore, the element 1 can realize the resistance switching. As a
result, the high-speed switching using a pulse of about 100 ns can
be executed.
[0105] The present element 1 includes two hafnium oxide layers,
each having a different oxygen defect concentration, wherein one of
them having high oxygen defect concentration is the resistance
change layer 15, while the other one having low oxygen defect
concentration is the high-oxygen layer 16. However, the number of
layers having different oxygen defect concentration may be three or
more, and the oxygen defect concentration may continuously be
changed.
Second Embodiment
[0106] In the first embodiment described above, the present element
1 includes the resistance change layer 15 and the high-oxygen layer
16, these layers being made of the same metal oxide but having
different oxygen defect concentration. However, the resistance
change layer 15 and the high-oxygen layer 16 may be made of a
different metal oxide. The resistance change of the variable
resistive element appears since oxygen atoms are collected or
diffused by the electric field near the interface between the
electrode and the variable resistor. Therefore, it is more
preferable that the high-oxygen layer 16 is made of a different
oxide or oxynitride having free energy of formation of oxide higher
than that of the oxide or oxynitride forming the resistance change
layer 15. This structure facilitates the oxygen transfer from the
high-oxygen layer 16 to the resistance change layer 15 during the
reset operation, whereby the reset current, which is difficult to
control only by the limitation of current by the transistor, can be
reduced.
[0107] Ellingham diagram in FIG. 6 illustrates a temperature change
in free energy of formation of oxide per 1 mol of oxygen molecule
of the respective metal oxides. In the graphs in FIG. 6, the value
of the leftmost free energy (having the lowest temperature)
indicates the standard free energy of formation. It is found from
FIG. 6 that hafnium oxide is a metal oxide whose standard free
energy of formation is lower than that of aluminum oxide.
[0108] In the present embodiment, in the present element 1, the
hafnium oxide HfO.sub.X film is used as the resistance change layer
15, and an aluminum oxide AlO.sub.Y film with less oxygen defect is
used, instead of the hafnium oxide HfO.sub.Y film with less oxygen
defect, as the high-oxygen layer 16. Thus, a variable resistive
element 2 is formed. This element is referred to as "the present
element 2" below.
[0109] When the oxygen defect concentration is compared among metal
oxides having different oxidation number, it is impossible to
simply compare the oxygen defect concentration based upon the
oxygen composition ratio per one metal element. It is necessary
that the stoichiometric composition of the respective metal oxides
is considered, and the oxygen defect concentration is compared
based upon the ratio obtained by dividing the oxygen composition
ratio by the oxygen composition ratio in the stoichiometric
composition.
[0110] In the present embodiment, the stoichiometric composition
ratio of hafnium oxide is HfO.sub.2, and the stoichiometric
composition ratio of aluminum oxide is Al.sub.2O.sub.3. Therefore,
the value obtained by dividing the oxygen concentration X per one
Hf of the HfO.sub.X film forming the resistance change layer 15 by
2, and the value obtained by dividing the oxygen concentration Y
per one Al of AlO.sub.Y film forming the high-oxygen layer 16 by
3/2 are compared, and the oxygen concentration of the hafnium oxide
(HfO.sub.X) forming the resistance change layer 15 and the oxygen
concentration of aluminum oxide (AlO.sub.Y) forming the high-oxygen
layer 16 are adjusted in order that X/2<2Y/3 is established.
[0111] FIG. 7 illustrates a cumulative frequency distribution of a
resistance value after the set operation and a resistance value
after the reset operation after the 1000 bits of elements 2
according to the present invention, each of which has the hafnium
oxide HfO.sub.X2 film (X2=1.8) as the resistance change layer 15,
and the aluminum oxide (AlO.sub.Y) film (Y=1.5) as the high-oxygen
layer 16, underwent the switching operation ten times. As in the
first embodiment, the experiments of the resistance switching were
carried out by using a memory cell illustrated in the equivalent
circuit diagram in FIG. 5 and having a transistor T connected in
series, and voltage pulse Vd was applied from the present element
2.
[0112] In the present embodiment, the drive current of the
transistor T was limited to 10 .mu.A, and the voltage pulse of +3.2
V was applied for 100 ns during the forming operation for forming
the filament path first. During the set operation for changing the
element from the high resistance state to the low resistance state,
the drive current of the transistor T was limited to 10 .mu.A, and
the voltage pulse of +2.5 V was applied for 100 ns. On the other
hand, during the reset operation for changing the resistance state
from the low resistance state to the high resistance state, the
gate of the transistor T was fully opened without the limitation of
current, and the voltage pulse of -1.7 V was applied for 20 ns. In
this case, the reset current flowing through the element during the
reset operation was about 120 .mu.A.
[0113] It is found from FIG. 7 that the present element 2 realizes
stable resistance switching with the state in which the set current
is limited to be not more than 10 .mu.A. The present element 2 can
also realize high-speed switching of about 100 ns. It is also found
that, compared to the variable resistive element 1 using hafnium
oxide as the high-oxygen layer 16 illustrated in FIG. 4, the
variation in the resistance values of the variable resistive
element can be reduced.
Third Embodiment
[0114] FIG. 8 illustrates a non-volatile semiconductor memory
device using the present element 1 or 2 described above. FIG. 8 is
a circuit block diagram illustrating a schematic configuration of a
non-volatile semiconductor memory device 20 (hereinafter referred
to as "present device 20" as needed) according to one embodiment of
the present invention. As illustrated in FIG. 8, the present device
20 includes a memory cell array 21, a control circuit 22, a voltage
generating circuit 23, a word-line decoder 24, a bit-line decoder
25, and a source-line decoder 26.
[0115] The memory cell array 21 includes a plurality of memory
cells, each of which includes the variable resistive element R, in
at least one of a row direction and a column direction in a matrix.
The memory cells belonging to the same column are connected by a
bit line extending in the column direction, and the memory cells
belonging to the same row are connected by a word line extending in
the row direction. The memory cell array is the one illustrated in
an equivalent circuit diagram in FIG. 9, for example. The memory
cell array 21 illustrated in FIG. 9 is a 1T1R memory cell array in
which a unit memory cell includes a transistor T serving as a
current limiting element. One electrode of the variable resistive
element R is connected to one of a source or a drain of the
transistor T in series to form a memory cell C. The other
electrode, not connected to the transistor T, of the variable
resistive element R is connected to bit lines BL1 to BLm (m is a
natural number) extending in the column direction (in the vertical
direction in FIG. 9), the other one of the source and the drain of
the transistor T that is not connected to the variable resistive
element R is connected to source lines SL1 to SLn (n is a natural
number) extending in the row direction (in the lateral direction in
FIG. 9), and the gate terminals of the transistors are connected to
word lines WL1 to WLn extending in the row direction. Any one of
selected word line voltage and non-selected word line voltage is
applied through the word line, any one of selected bit line voltage
and non-selected bit line voltage is applied through the bit line,
and any one of selected source line voltage and non-selected source
line voltage is applied through the source line, wherein these
voltages are independently applied. With this process, one or a
plurality of memory cells, which are targets of the action
designated by an address input from the outside such as a
programming action, erasing action, reading action, and forming
process, can be selected.
[0116] The variable resistive element R forming the memory cell C
may be either one of the present elements 1 and 2. The structure of
the variable resistive element R is not particularly limited, so
long as the variable resistor 13 including two layers that are the
resistance change layer 15 and the high-oxygen layer 16 is
sandwiched between the electrodes 11 and 12.
[0117] The control circuit 22 controls the operation of each
memory, such as the programming action (an action for decreasing
the resistance: set operation), the erasing action (an action for
increasing the resistance: reset operation), and reading action of
the memory cell array 21, and controls the forming process.
Specifically, the control circuit 22 controls the word-line decoder
24, the bit-line decoder 25, and the source-line decoder 26 based
upon an address signal inputted from an address line, a data input
inputted from the data line, and a control input signal inputted
from an a control signal line, thereby controlling the action of
each memory in each memory cell and the forming process. Although
not illustrated in FIG. 8, the control circuit 22 has a function of
a general address buffer circuit, a data input/output buffer
circuit, and a control input buffer circuit.
[0118] The voltage generating circuit 23 generates the selected
word line voltage and non-selected word line voltage necessary for
selecting the target memory cell during each of the programming
action (an action for decreasing the resistance: set operation),
the erasing action (an action for increasing the resistance: reset
operation), and the reading action of the memory, and the forming
process of the memory cell, and supplies the resultant to the
word-line decoder 24. The voltage generating circuit 23 also
generates the selected bit line voltage and non-selected bit line
voltage, and supplies the resultant to the bit-line decoder 25. The
voltage generating circuit 23 also generates the selected source
line voltage and non-selected source line voltage, and supplies the
resultant to the source-line decoder 26.
[0119] When the target memory cell is inputted to the address line
to be designated during each of the programming action (an action
for decreasing the resistance: set operation), the erasing action
(an action for increasing the resistance: reset operation), and the
reading action of the memory, and the forming process of the memory
cell, the word-line decoder 24 selects the word line corresponding
to the address signal inputted to the address line, and applies the
selected word line voltage and the non-selected word line voltage
to the selected word line and to the non-selected word line,
respectively.
[0120] When the target memory cell is inputted to the address line
to be designated during each of the programming action (an action
for decreasing the resistance: set operation), the erasing action
(an action for increasing the resistance: reset operation), and the
reading action of the memory, and the forming process of the memory
cell, the bit-line decoder 25 selects the bit line corresponding to
the address signal inputted to the address line, and applies the
selected bit line voltage and the non-selected bit line voltage to
the selected bit line and to the non-selected bit line,
respectively.
[0121] When the target memory cell is inputted to the address line
to be designated during each of the programming action (an action
for decreasing the resistance: set operation), the erasing action
(an action for increasing the resistance: reset operation), and the
reading action of the memory, and the forming process of the memory
cell, the source-line decoder 26 selects the source line
corresponding to the address signal inputted to the address line,
and applies the selected source line voltage and the non-selected
source line voltage to the selected source line and to the
non-selected source line, respectively.
[0122] FIG. 10 is a sectional view schematically illustrating one
example of a device structure of the memory cell array 21. The
memory cell array 21a whose cross-section is illustrated in FIG. 10
is the 1T1R memory cell array using the present element 1 for the
memory cell. In the memory cell array 21a, the first electrode 14
extends in the column direction (in the lateral direction in FIG.
10) to form the bit line BL, and the resistance change layer and
the high-oxygen layer 16 similarly extend in the column direction.
The contact plug that connects the transistor T formed in the lower
layer via the island-like metal wiring 31 and contact plug 32 is
the second electrode 12 connected to the resistance change layer
15. The variable resistive element 1 including the first electrode
14, the resistance change layer 15, the high-oxygen layer 16, and
the second electrode 12 is formed on the contact region (element
formation region) where the second electrode 12 is in contact with
the resistance change layer 15.
[0123] FIG. 11 is a perspective view illustrating another example
of the device structure of the memory cell array 21. A memory cell
array 21b illustrated in FIG. 11 is a three-dimensional memory cell
array in which the present elements 2 using HfO.sub.X as the
resistance change layer 15 and AlO.sub.Y as the high-oxygen layer
16 are arranged in X direction, Y direction, and Z direction. The
memory cell array 21b is formed such that the inner peripheral
sidewall of a through-hole that penetrates the stacked structure of
the first electrode 14 (here, Ti) and an interlayer dielectric film
33 is covered successively by the high-oxygen layer 16 and the
resistance change layer 15, and the through-hole is filled with the
second electrode 12 (here, TiN).
[0124] FIG. 11 illustrates the cross-sectional structure on XZ
section or YZ section including the axis of the through-hole. The
second electrode 12 is connected to a diffusion area 34 forming the
drain of the transistor T formed on the substrate 10. Each of the
sheet-like first electrodes 14 becomes the bit line extending in
the X direction and Y direction. The position of the variable
resistive element in the X direction and Y direction is specified
by using the transistor T, and the position of the variable
resistive element in the Z direction is specified by selecting the
bit line. With this, the action of the memory cell of the
three-dimensionally arranged variable resistive element on any
position can be executed.
[0125] The detailed circuit structure of the control circuit 22,
the voltage generating circuit 23, the word-line decoder 24, the
bit-line decoder 25, and the source-line decoder 26 can be realized
by using a known circuit structure, and the device structure of
these components can be manufactured by using a known semiconductor
manufacturing technique. Therefore, the detailed circuit structure,
the device structure, and the manufacturing method will not be
described here.
[0126] According to the present invention, the variable resistive
element includes the high-oxygen layer 16, whereby the variable
resistive element that can perform a stable switching operation
with low voltage and small current can be realized, and a
large-capacity low-power consumption non-volatile semiconductor
memory device using the variable resistive element can be
realized.
Other Embodiment
[0127] Other embodiments will be described below.
[0128] (1) Although the variable resistive element having the
element structure illustrated in FIG. 1 is described as one example
in the first and second embodiments described above, the present
invention is not limited to the element having such a structure.
The present invention is applicable to a variable resistive element
having any structure, as long as the variable resistor 13 includes
two layers that are the resistance change layer 15 and the
high-oxygen layer 16, and the composition is adjusted such that the
oxygen concentration of the high-oxygen layer 16 is higher than
that of the resistance change layer 15. The present invention is
not limited by the thickness or oxygen concentration of the
resistance change layer 15 and the high-oxygen layer 16, or the
element area.
[0129] (2) The high-oxygen layer 16 is inserted between the
resistance change layer and the electrode provided on the other
side of the electrode with which the resistance change layer 15 is
in contact. However, as illustrated in a variable resistive element
3 in FIG. 13, the high-oxygen layer 16 may be inserted as a layer
other than the resistance change layer of the variable resistor
including a plurality of layers.
[0130] The considered examples in which the variable resistor 13
includes a layer other than the high-oxygen layer 16 and the
resistance change layer 15 include a structure in which a tunnel
insulation film is inserted between the first electrode 12 and the
resistance change layer 15 in order to provide a function as the
non-linear current limiting element to the element, and a structure
in which a buffer layer that suppresses a rapid increase in the
current flowing between the electrodes of the variable resistive
element upon the completion of the forming process is inserted in
order to reduce the variation in filament path, formed by the
forming process, among the elements. When the absolute value of the
free energy of the formation of oxide of the metal forming the
electrode is larger than the absolute value of the free energy of
the formation of oxide of the metal oxide layer that is in contact
with the electrode, a part of the electrode is oxidized, so that an
oxide film or oxynitride film of the electrode is formed between
the metal oxide layer and the electrode. Alternatively, when the
first electrode 14 or the second electrode 12 is a lower electrode,
the oxide film or oxynitride film of the lower electrode may be
formed on the surface of the lower electrode after the formation of
the lower electrode, due to the manufacturing process.
[0131] In the embodiments described above, the resistance change
layer 15 is in contact with the second electrode 12. However,
another layer may be inserted between the resistance change layer
15 and the second electrode 12. The variable resistive element
according to the present invention can be realized, so long as the
thickness of the tunnel insulation film, the buffer layer, or the
natural oxide film inserted between the resistance change layer and
the second electrode 12 is thin, and the interface with the
electrode keeps the state of generating energy gap of conductive
carriers, such as a schottky junction.
[0132] (3) In the third embodiment, the present device 20 is
applicable to any memory cell array including a plurality of memory
cells arranged in a matrix, so long as the variable resistive
element according to the present invention including metal oxide as
the resistance change layer 15 and further including the
high-oxygen layer 16 is used as each of the memory cells. The
present invention is not limited by the structure of the memory
cell array 21 or the circuit structure of the other circuits such
as the control circuit or the decoders. In particular, the memory
cell array 21 may be a 1R memory cell array that does not contain a
current limiting element in a unit memory cell illustrated in FIG.
14, or may be a 1D1R memory cell array including a diode, serving
as a current limiting element, in a unit memory cell, in addition
to the 1T1R memory cell array 21 illustrated in FIG. 9. In the 1D1R
memory cell array, one end of the diode and one electrode of the
variable resistive element are connected in series to form a memory
cell, any one of the other end of the diode and the other electrode
of the variable resistive element is connected to the bit line
extending in the column direction, and the other one is connected
to the word line extending in the row direction. In the 1R memory
cell array, both electrodes of the variable resistive element are
respectively connected to the bit line extending in the column
direction and to the word line extending in the row direction.
[0133] (4) The present device 20 includes the source-line decoder
26 for selecting the source lines SL1 to SLn, wherein each source
line is selected to allow the voltage necessary for the operation
of the memory cell to be applied. However, the source line may be
shared by all memory cells, and a ground voltage (fixed potential)
may be supplied to the source line. Even in this case, the voltage
necessary for the operation of the memory cell can be supplied by
selecting each of bit lines BL1 to BLn through the bit-line decoder
25.
[0134] The present invention is applicable to a non-volatile
semiconductor memory device, and more particularly applicable to a
non-volatile semiconductor memory device including a non-volatile
variable resistive element whose resistance state is changed due to
application of voltage, the resistance state after the change being
retained in a non-volatile manner.
[0135] Although the present invention has been described in terms
of the preferred embodiment, it will be appreciated that various
modifications and alternations might be made by those skilled in
the art without departing from the spirit and scope of the
invention. The invention should therefore be measured in terms of
the claims which follow.
* * * * *